Method of constructing propellers



Oct. 3, 1961 J. E. LYNN ET AL METHOD OF CONSTRUCTING PROPELLERS 5 Sheets-Sheet 1 Filed April 24, 1957 n j l 1 JACK E. LYNN SAMUEL P. ROBINSON INVENTORS ATTURNEYS Oct.

Filed April 24, 1957 J- E. LYNN ET AL METHOD OF CONSTRUCTING PROPELLERS FIG. 7

5 Sheets-Sheet 2 FIG.4

JACK E. LYNN SAMUEL P. ROBINSON INVENTORS ldTTORNEYS Oct. 3, 1961 J. E. LYNN ET AL 3,002,266

' JACK E. LYNN e s SAMUEL R ROBINSON NNNNNNN RS 3,002,266 METHOD OF CONSTRUCTING PROPELLERS Jack E. Lynn, Cromwell, Comm, and Samuel P. Robinson, State College, Pa., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Apr. 24, 1957, Ser. No. 654,976 11 Claims. (Cl. 29-1568) where v is the cavitation index, P is the local pressure in pounds per square foot, P is the vapor pressure of the liquid in pounds per square foot, ,0 is the massidensity of the fluid in slugs per cubic foot and V is the free stream velocity in feet per second, i.e., the relative velocity between the body involved and the undisturbed fluid;

Empirical methods have increased the efficiency of marine propellers to respectable values and it has been found that the existence or absence of a limited amount of cavitation does not, under design operating conditions, have a substantial effect on the efliciency of the propeller. Prior art propellers have achieved acceptable propulsion efliciencies and are satisfactory when considered solely from the standpoint of propelling the vehicle. From a detectable noise standpoint, the prior art propellers are poor in that with even moderate forward velocities and/or shallow operating depths substantial external noise is produced.

The noise produced by torpedos, submarines and surface vessels is of particular importance to the designer of each of these vehicles when consideration is taken of modern warfare methods. One method of detecting any of the above vehicles is through acoustic methods in which the external noise output of the vehicle supplies the parameter which is observed and the higher the external noise level of the vehicle the farther away detection can be performed.

An acoustic torpedo is used by Way of an example but it is to be noted that the discussion applies equally as well to a submarine (which is in all operating aspects a manned torpedo) and in some aspects to surface craft.

Both the self noise and the externally detectable noise of an acoustic torpedo is of utmost importance to the torpedo designer. The higher the external detectable noise the easier, and at longer ranges, the torpedo can be detected and thus the major advantage of stealth is reduced or eliminated. The self noise of the torpedo due to its internal machinery, gears, propulsion unit and the like causes a reduction in the torpedos acquisition range. It is therefore evident from-the above brief discussion that noise must be kept to a minimum during the entire search trajectory of an acoustic torpedo.

The most important aspect of cavitation, from a noise standpoint, is that the very first beginnings of cavitation produce a very large increase of, for example, 20 to 30 db in noise. Therefore, from a quiet operating aspect no cavitation on the propellers can be tolerated. Since the inception of cavitation results in large increases in ice 2 noise the inception condition and large increases in noise can therefor be considered as the same item.

By reference to the cavitation parameter as given in Equation 1, it may be seen that the incipient cavitation index is essentially a relationship between the operating depth and the speed of the vehicle. For a given incipient cavitation index, a determination of either of the parameters automatically determines the other. The incipient cavitation index, i.e., that operating index at Which cavitation first begins is determined by the design criteria employed in the actual design of the propeller and by the maintaining of a very high degree of accuracy or conformance to the theoretical configuration throughout the manufacturing process.

The importance of this will be readily appreciated from the following example. A conventional torpedo propeller having an incipient index of cavitation of 2.0 and operating at a depth of fifty feet will cavitate (become noisy) at speeds in excess of about 30.6 knots and when travelling just below the surface, it will cavitate at a speed in excess of about 19.2 knots. A propeller may be constructed in accordance with the present invention having an incipient index of cavitation of about 0.5 that when travelling at a depth of about feet will not cavitate (become noisy) until it exceeds a speed of about 61 knots and that can travel just below the surface without cavitation until it exceeds a speed of about 38.5 knots.

It is believed that heretofore the prior art has-been I unable to consistently secure improved cavitation characteristics on torpedo propellers due to the inability to consistently secure or maintain the necessary tolerances and configuration required for a propeller to have a low incipient index of cavitation. Additionally, the prior art method of casting the propeller as a unit and then hand finishing the blades is substantially limited to four or five bladed propellers and is an expensive and time consuming operation with a high percentage of rejects or unsatisfactory propellers with regard to cavitation. Still further, it is extremely difiicult to properly inspect such propellers to insure the maintenance of design tolerances.

An object of the invention is to provide an improved method of making propeller blades having an airfoil crosssection wherein the desired airfoil cross-section may be repeatedly reproduced with a high degree of accuracy. Still another object of the invention is to provide an improved method of producing torpedo propeller blades in quantity wherein such blades may be more accurately for-med and may be quickly assembled to form a propeller having blades that are accurately positioned and orientated.

A further object of the invention is to provide an improved method of making torpedo propellers having improved cavitation characteristics.

A still further object of the invention is to provide an improved method of producing torpedo propellers in quantity wherein such propellers may be more perfectly manufactured thereby resulting in propellers having a lower index of cavitation, and consequently less tendency to be noisy.

These, and other features of the invention, together With their incident advantages, will be more readily appreciated and understood from the following detailed description of the preferred embodiment and the method of production described hereinafter and selected for purposes of illustration and shown in the accompanying drawings in which:

FIGURE 1 is a rear view in perspective of a four bladed propeller constructed in accordance with the present invention.

FIGURE 2 is a perspective view (with a section broken away.) of the cylindrical central element on which the blade elements are mounted.

FIGURE 3 is a rear view in perspective of a blade element comprised of a bladeand root section forming one section of a four bladed propeller, final form being shown in solid lines and an optional initial pre-cast form or blank being shown in phantom lines as a modification.

FIGURE 4 is a perspective view of -a blank in an intermediate stage of construction preparatory to forming a blade and root section of a ten bladed propeller.

FIGURE 5 is a plan view of a plurality of partially formed blanks assembled in a jig preparatory to forming the inner surfaces of the root sections of each blade elements.

FIGURE 6 is a sectional view taken on line 6-6 of FIGURE 5.

FIGURE 7 is a sectional view of a blank forming a partially cast blade and root section.

FIGURE 8 is a front view of a ten bladed propeller constructed in accordance with the present invention.

FIGURE 1 of the drawings shows a propeller of the conventional four-bladed type constructed according to the present invention. Each blade 10 is integral with a root section 11 that, when mounted on the spool-like cylindrical central element 12 (FIGURE 2) forms the major portion of the propeller hub. As best shown in FIGURE 3, each root section 11 is characterized by an outer surface 13 and by oppositely disposed side surfaces 14 adapted to mate with a similar side surface 14 of an adjoining root section and by cylindrical and transverse radial inner surfaces adapted to mate with the outer surfaces of the cylindrical central element, as will be more thoroughly described. With reference now to FIG- URE 3, each blade root section 11 is removably attached to the cylindrical element 12 as by bolts 15 or the like and a spline 16, keyway or the like is provided on the inner surface of the cylindrical element 12 to attach the propeller in non-rotatable relationship with a propeller shaft (not shown).

In accordance With one embodiment of the invention, each blade 10 and root section 11 may be formed from a blank 17 shown in FIGURE 4 and then removably at- 'tached to the cylindrical central element 12 to constitute a unitary structure. A blade element 18 which for convenience of illustration is shown in its ultimate form as one blade of the four bladed propeller shown in FIGURE *1, comprises a blade 10 of any suitable material, such as for example, stainless steel, plastic or the like of airfoil cross'section integral with a root section 11 which comprises a portion of the propeller hub. The outer surface 13 of each root section comprises a portion of the conically shaped outer surface of the hub which is formed by the assembly of each blade element 18 on the central element 12 as will be more fully discussed hereinafter. The inner surface of each root section 11 is cylindrical in shape and is provided with inwardly extending cylindrical mating surfaces 19 at the forward and rearward portions which mate with the outer central surface 21 of the cylindrical central element 12. To facilitate proper mating of the inner surfaces 19 of the root sections 11 and the outer surface 21 of the cylindrical central element 12 the central portion of the inner surface of each root section is relieved to form a surface 22 having a diameter slightly greater than the mating surfaces 19' which surfaces should have a diameter substantially the same as the 'diameter of the central surface 21 of the central oylindr'ical element. The front surface 24 and rear surface 23 of each root section are cylindrically recessed Each blade and root section comprising a blade element 18 is preferably formed from a metal or plastic blank 17 comprised of an alloy steel, stainless steel or the like. The initial dimensions of each blank 17 depend on the size configuration and number of blades used to form a completed propeller which may have from two to fifteen or more blades. In order to establish reference surfaces for the formation of the radial side surfaces 14 of each root section the front and rear surfaces 29 and the side surfaces 32 of blank 17 may be milled or maohined substantially parallel with its respective oppositely disposed side. The formation of a reference surface or surfaces thereby allows the radial or angular surfaces 14 to be accurately formed by removal of the material 36 shown in phantom in FIGURE 4 such that they are precisely disposed at a desired predetermined angle preferably having an apex 33. The angular displacement of the radial sides 14 will of course depend on the number of blades it is desired to provide on the propeller.

To provide a reference surface for alignment of the blank 17 preparatory to forming the inner surfaces 19, 22 and groove 25 on the root section as will be more thoroughly discussed hereinafter, a predetermined portion 34 of the apex 33 formed by the radial side surfaces 14 is removed as by milling or the like. The reference surface 35 formed thereby may be of any convenient configuration such as cylindrical or flat as shown in FIG- URE 4. The longitudinal location of the reference surface 35 may be accurately, consistently and conveniently positioned by forming the surface 35 coincident with a cord or the like of a suitable predetermined length of the angle formed by radial sides 14. The length of the cord is not critical but the coincidence of reference surface '35 therewith is essential to the proper alignment of the blank 17 to insure interchangeability of the blade ele' ments 18.

As shown in FIGURE 5 a plurality of blanks 17 (in this case 10) corresponding to the number of blades the propeller is to have, having been previously prepared in the manner described hereinabove', are assembled in a jig 37 or holding device adapted to be mounted on a lathe, milling machine or the like.

'As shown in FIGURE 5 and FIGURE 6 the jig 37 or holding device may be comprised of a sufficiently rigid and flat circular front plate 38 and rear plate 39 held in fixed oppositely disposed relationship by an annular side wall 41 adapted to allow insertion and removal of the blanks from the jig 37. The front plate and rear plate may be provided with axial openings 42 sufiiciently large to allow access to the central portion of the assembled blanks and set screws 43 adapted to register with each blank maybe provided to lock the blanks in their desired position once they have been properly aligned.

It is to be understood that the jig or holding device described hereinabove may be constructed other than in the manner described without departing from the scope -or intent of the invention, it being the function of the jig to provide a means for suitably holding the partially formed blanks whereby they maybe properly aligned and the inner surfaces of the root sections formed.

Each blank is accurately positioned by means of the reference surfaces 35, a feeler gauge, indicator or the like being used to indicate when each reference surface 35 is properly positioned, such as for example, in registry with each other reference surface whereby a substantially perfect circle, polygon'or the like is formed depending on the number of blanks and the type of reference surface 35. If desired the positioning of the blanks may be performed after the jig is mounted in a lathe, milling machine or the like thereby insuring that the axial location of each blank is substantially identical with regard to a common axis which axis corresponds with the longitudinal axis-of the lathe or the like Wherebyerrors in the concentricity of surfaces 19, 22 and'g'rooves 25 are virtually impossible. After the blanks have been positioned and securely locked in position in the jig 37 by means of set screws 43 or the like, the inner surfaces 19, 22 and grooves 25 of the root portion 11 are formed by mounting the jig in a lathe, milling machine or the like and removing the center portion 44 of the blanks shown in phantom in FIGURE 5.

It may now be obvious that the utilization of the cylindrical and radial surfaces concentric about a central axis allows all mating surfaces to be quickly and conveniently formed in a single operation with a high degree of accuracy, thus insuring that each surface will be properly located and will mate with its respective mating surface on the cylindrical central element and also provides accurate reference surfaces that will allow the airfoil cross section of the blades and the outer root surface 13 to be formed and orientated with a degree of precision not heretofore obtainable. The normally unexposed surfaces 14--19 and grooves 25 of the root section and the mating surfaces 2127 of the cylindrical central element 12 when formed in accordance with the foregoing method can be consistently held to a'tolerance of .0005 inch which in turn allows the formation of blade and root surfaces having a degree of accuracy not heretofore obtainable.

As has been pointed out hereinabove, the procedure of first forming the inner surfaces of the root sections substantially in the manner previously described is essential to a precise formation, location and orientation of the solid airfoil blades. Solid airfoil sections are most conveniently formed on profile rnillers and such may be accurately accomplished provided the'blanks to be formed can be precisely positioned and/or orientated with regard to the cutting element. Similarly, the tolerance, as used in its broadest sense, of this milling operation cannot be accurately determined unless the blank can be accurately positioned and/or orientated in a pantograph or the like adapted for inspection of the airfoil surface.

It may now be obvious that in view of the configuration of airfoil sections and variations thereof, such sections cannot be quickly and easily produced in large quantities with any degree of precision unless the requirements discussed immediately hereinabove are met. To meet these requirements, and in accordance with the present invention, the blade and exposed surface'13 of the root section of a plurality of propeller blades may be quickly, easily and consistently positioned and orientated both on a profile miller or the like and an inspection device with the requisite degree of accuracy in the following manner.

Since each partially completed blank has a common axis, which axis is coincident with the longitudinal axis of the propeller shaft, each such blank or a succeeding series of blanks may be positioned and/ or orientated with the same high degree of accuracy by providing a blade holding fixture or mount comprised of a portion of the central element 12 shown in FIGURE 2 and attaching a partially completed blank to this portion of the central element. It is then only necessary to attach the blade holding fixture or mount (one may be provided for the profile miller or the like and one may be provided for the inspection device) in the conventional manner as may be desired or convenient. Since all blanks, whether preformed, partial preformed or not pre-formed at all for a particular propeller or a type of propeller are substantially identical after formation of the root surfaces it may now be obvious that once the mounting means has been properly positioned, no undesirable error can result from removal or transferal of the same or a plurality of blanks from one mount or from one mount to another mount. This is of particular importance since if a propeller is to have a low cavitation index the propeller blades must he held to very close tolerances and it is therefore necessary that the blades be capable of inspection with a high degree of accuracy.

The cylindrical central element 12 may be produced independently of the blade and root sections with the same degree of accuracy as the surfaces on the root section of each blade since all critical surfaces are substantially cylindrical or radial and concentric about a single longitudinal axis. As shown in FIGURE 2 the cylindrical central element 12 is preferably provided with a cylindrical outer surface 21 substantially concentric about its longitudinal axis and adapted for mating relationship with the inner surfaces 19 of the root section. There is also provided at each extreme end annual rings 28 integral with the main body section 45 having a diameter greater than the diameter of the cylindrical element outer surface 21 and adapted for mating relationship with the groove 25 of the root sections. Counterbored radial passages 46 may be provided in the main body section 45 to receive bolts 15 or the like to retain the completed blade element 18 on the cylindrical central element 12.

A forward portion 47 of the inner surface of the cylindrical element may be provided with a diameter approximately equal to the diameter of the propeller shaft and the remaining rearward portion 48 may be provided with a diameter sufficiently large to receive a spline adapted to be pressed into abutting relationship with the radial surface 49. Passages '51 may be formed in the spline 16 to register with the radial passages 46 in the main body portion 45 of these passages may be formed after the spline has been pressed into position. The forward surface 47 performs'the function of preventing wobble of the propeller on the shaft and the spline 16 being securely fixed with relation to the cylindrical central element 12 by means of the connecting bolts 15 and meshed with the propeller shaft, holds the propeller in fixed relationship with the propeller shaft. It is to be noted that other conventional methods of attaching the central element 12 will occur to those experienced in the art and are intended to be included within the scope of the invention.

Although the preferred method of forming propeller blades and root sections in accordance With the present invention have been described hereinabove, it is to be understood that other approaches may be used, such as for example, casting or partial casting to approximate form to eliminate the majority of the machining operation with regard to the blade and/or the root section. Although present day casting techniques are satisfactory for most purposes, such techniques do not possess sufficient accuracy to allow casting of the finished propeller blade and root section to the required degree of accuracy, such as for example, .001 to .0005 of an inch. However, if the utmost in precision is not required casting or partial casting to approximate form may be utilized. If, for example, the blade element 18 as shown in FIGURE 3 is cast close tolerances and a sufficiently thick envelope 52 of material is provided on the blade or airfoil section a high degree of accuracy may still be obtained in the following manner. If it is assumed, for example, that either the front or rear surfaces 24--23 of the root section are true and these surfaces are used as reference surfaces, or, alternately, a flat reference surface may be machined on a boss cast for example at the tip of the blade, the side surfaces 14 may be formed at the desired angle and orientation. In view of the assumption of at least one true reference surface, it may now be obvious that the true orientation of the blade is destroyed by the formation of the proper surfaces on the root section. The envelope 52 of material on the blade section is therefore necessary for a finishing operation similar to that described hereinbefore whereby the blade 10 is in effect properly positioned and orientated with regard to the root section. It is also to be understood that, if desired,

the blade section may be cast in the form of 'a solid reotangular block rather than approximating the final form of the blade. 1

I-f preferred, an alternate construction as shown in FIGURE 7 may be utilized wherein the blade 10 is cast with an envelope -2 as previously described and the root section is cast in the form of a rectangular block 53- instead of its approximate final form. It may now be obvious that the sides 54 of the block may be machined true thereby providing true reference surfaces whereby the root section and blade in every respect can be formed with'the requisite degree of accuracy. It is to be noted that the procedure described last hereinabove has the advantage of eliminating all but the final cut or cuts in forming the blade while retaining all of the advantages 'of the present invention. The root section of such a partially cast blade or blank is preferably formed in substantially the same manner as for a blank 17 as previously described, the final cut or cuts made on the blade and the blade inspected in the manner previously described to insure the requisite formation and orientation of the blade.

The present invention has especial utility in the construction of propellers having a large number of blades, such as for example, a ten bladed propeller as shown in FIGURE 8. Propellers having an increased number of blades may in general be designed according to the principles applicable to the design of a conventional two bladed or four bladed propeller but it is at this point that similarityto the prior art ceases. However, such a multibladed torpedo propeller, such as for example, one having fifteen blades, cannot be cast, machined and inspected with any degree of accuracy, if at all, and the proper orientation and location of one blade with respect to any other'blade is essential to realize design performance of the propeller.

Further, for certain applications dependent upon the design, size and requirements of the propeller, the proximity of one blade to another requires that each root section be skewed to conform to the surface of the central element and the appropriate angle of attack of each blade and it is in this instance that prior art methods are singularly deficient for quantity production of a propeller having a low index of cavitation.

The propeller blade, propeller construction and the method of construction according to the present invention, although applicable to marine propellers in general is especially useful for the manufacture of torpedo propellers that not only are eflicient and will propell a torpedo at the desired speed, but will do so without cayitating, which is to say, create noise, at maximum speed and/ or minimum operating depth. Heretofore the inability to produce such a propeller is believed to be due to the inability to hold all tolerances on such a propeller Within the very small limits which have been found necessary to secure improved cavitation characteristics.

In accordance With the present invention, the location and orientation of the blades and the tolerances on both the propeller blades and the propeller hub can be held to tolerances smaller than that heretofore obtainable in addition to allowing the use of metals such as for example, stainless steel, plastics and the like. Still further, the present invention provides improved manufacturing procedures for the construction of propellers while maintaining all of the advantages hereinbefore discussed in addition to the removability and complete interchangeability features of the blades without any undesirable results.

While the present invention has been described in its preferred embodiment, it is realized that modifications may be made, and it is desired that it be understood that no limitations upon the invention are intended other than may be imposed by the scope of the appended claims.

Having now disclosed our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

l. The method of manufacturing marine propeller blade elements of the type having integral blade and root sections which comprises: forming first and second side surfaces at a predetermined .angleon the root section;

arranging a plurality of blade elements having said first and second side surfaces in contiguous relationship to form. an annulus and positioning said blade elements about a common axis; forming inner cylindrical surfaces concentric with said annulus on said positioned blade elements; thereafter, using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forming a blade section from each of said blade elements.

2. The method of manufacturing marine propeller blade elements of the type having integral blade and root sections which comprises: forming first and second side surfaces at a predetermined angle on the root section; arranging a plurality of blade elements having said first and second side surfaces in abutting relationship to form an annulus and positioning said blade elements whereby the location of each said blade element is substantially identical with regard to a common axis; forming inner cylindrical surfaces concentric with said annulus while said blade elements are in said fixed position; thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cy-.- lindrical surfaces and separately forming a blade section from each of said blade elements.

3. The method of manufacturing marine propeller blade elements of the type having integral blade and root sections which comprises: forming a first reference surface; forming first and second side surfaces at a predetermined angle to form a wedge shaped root section; arranging a plurality of blade elements having first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blade elements whereby the radial location of each said blade element is substantially identical with regard to a common axis; forming cylindrical inner root section surfaces concentric with said annulus while said blade elements are in said fixed position; thereafter separately forming a blade section and outer surface of the root section from each of said blade elements.

4. The method of manufacturing marine propeller blade elements of the type having integral blade and root sections which comprises: forming a first reference surface; forming first and second side surfaces at a predetermined angle to each other and with reference to said reference surface to form a Wedge shaped root section; arranging a plurality of blade elements having said first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blade elements whereby the radial location of each said blade element is substantially identical with regard to a common axis; thereafter forming cylindrical inner root section surfaces concentric with said annulus while said blade elements are in said fixed position; removably assembling each blade element and a base having a portion adapted for mating relationship with the inner surfaces of said root section, thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forming a blade section from each of said blade elements.

5. The method of manufacturing marine propeller blade elements of the type having integral blade and root sections which comprises: forming a first reference sur face; forming first and second side surfaces at a predetermined angle to each other and with reference to said reference surface to form a wedge shaped root section; arranging said plurality of blade elements having a first and second side surface in contiguous relationship to form an annulus and fixedly positioning said blade elements whereby the radial location of each said blade element is substantially identical with regard to a common axis; thereafter forming cylindrical inner root section surfaces concentric about said common axis while said blade elements are in said first position; thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forminga blade section and outer surface of the root section from each of said blade elements; and immovably aflixing said blade element so formed about said common axis.

6. The method of manufacturing marine propeller blade elements of the type having integral blade and root sections which comprises: forming a first reference surface; forrning first and second side surfaces at a predetermined angle to form a wedge shaped root section having an apex; forming a second reference surface between said first and second side surfaces and adjacent said apex; arranging a plurality of blade elements having said second reference surface, said first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blade elements by said second reference surface whereby the radial location of each said blade element is substantially identical with regard to a common axis; thereafter forming cylindrical inner root section surfaces concentric about said common axis while said blade elements are in said fixed position to provide reference surfaces for positioning the blade element; removably assembling each blade element and a base having a portion adapted for mating relationship with the inner surfaces of said root section and forming the blade and the outer surface of the root section of each blade element after using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces; and immovably alfixing said blade element so formed about said common axis.

7. The method of manufacturing marine propellers having individual blade elements comprised of a blade section of airfoil cross section integral with a root section forming a part of the propeller hub and adapted for attaching the blade element to a central element wherein each said blade element is formed from a blank which comprises the steps of: forming first and second side surfaces at a predetermined angle to form a wedge shaped root section; arranging a plurality of blanks having said first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blanks whereby the radial location of each said blank is substantially identical with regard to a common axis; thereafter forming inner root section surfaces concentric with said annulus while said blanks are in said fixed position; thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forming a blade section and outer surface of the root section from each of said blade elements; and assembling the requisite number of blade elements rigidly on a central element having an outer surface adapted for mating relationship with the inner surface of each root section.

8. The method of manufacturing marine propellers having individual blade elements comprised of a blade section of airfoil cross section integral with a root section forming a part of the propeller hub and adapted for attaching the blade element to a central element wherein each blade element is formed from a blank which comprises the consecutive steps of: forming a first reference surface on the blank; forming first and second side surfaces at a predetermined angle to form a wedge shaped root section; arranging a plurality of blanks corresponding to the number of blade elements for a propeller having said first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blanks whereby the radial location of each said blank is substantially identical with regard to a common axis; forming cylindrical inner root section surfaces concentric about said common axis while said blade elements are in said fixed position, said inner root surfaces providing reference surfaces for accurate positioning of the blade; thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forming a blade section and outer surface of the root section from each 10 v of said blade elements; and assembling the requisite number of blade elements in an immovable manner on a hollow central element having an outer surface adapted for mating relationship with the inner surfaces of each root section. 1

9. The method of manufacturing marine propellers having individual blade elements comprised of a blade section of airfoil cross section integral with a root section forming a part of the propeller hub and adapted for attaching the blade element to a central element wherein each said blade element is formed from a blank which comprises the steps of: forming first and second side surfaces at a predetermined angle to form a wedge shaped root section; arranging a plurality of blanks having said first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blanks whereby the radial location of each said blank is substantially identical with regard to a common axis; thereafter forming cylindrical inner root section reference surfaces concentric with said annulus while said blade'elements are in said fixed position; thereafter removably assembling each blank and a base having a portion adapted for mating relationship with the inner surfaces of said root section and thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forming a blade section and outer surface of the root section from each of said blade elements and assembling the requisite number of blade elements rigidly on a hollow central element having an outer surface adapted for mating relationship with the inner surfaces of each root section.

10. The method of manufacturing marine propellers having individual blade elements comprised of a blade section of airfoil cross section integral with a root section forming a part of the propeller hub and adapted for attaching the blade element to a central element wherein each said blade element is formed from a blank which comprises the consecutive steps of: forming a first reference surface on the blank; forming first and second side surfaces at a predetermined angle to form a wedge shaped root section; arranging a plurality of blanks corresponding to the number of blade elements for a propeller having said first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blade elements whereby the radial location of each said blank is substantially identical with regard to a common axis, forming a central cylindrical surface concentric about said common axis and first and second cylindrical reference surfaces having a diameter less than said central cylindrical surface and concentric about said common axis while said blanks are in said fixed position; thereafter using said inner cylindrical surfaces to accurately orient each of said blade elements with relationship to said cylindrical surfaces and separately forming a blade section and outer surface of the root section from each of said blade elements; and assembling the requisite number of blade elements immovably on a hollow cylindrical hub having an outer surface adapted for mating relationship with the inner surfaces of each root section.

11. The method of manufacturing marine propellers having individual blade elements comprised of a blade section of airfoil cross section integral with a root section forming a part of the propeller hub and adapted for attaching the blade element to a central element wherein each said blade element is formed from a blank which comprises the steps of: forming a first reference surface on the blank; forming first and second side surfaces at a predetermined angle to form a wedge shaped root section; forming a second reference surface between said first and second side surfaces; arranging a plurality of blanks corresponding to the number of blade elements for a propeller having a second reference surface and first and second side surfaces in contiguous relationship to form an annulus and fixedly positioning said blade elements by said second reference surface whereby the radial location of.

1 1 each said blank is substantially identical with'regard to a common axis; thereafter forming a central cylindrical surface concentric about saidcommon axis and first and second cylindrical reference surfaces having a diameter less than said central cylindrical surface and concentric about said common axis while said blanks are in said fixed position; removably assembling each blank and a base having a portion adapted for mating relationship with the inner reference surfaces of said root section and forming the blade and the outer surface of the root section of each blade element after formation of said inner root section surfaces; and assembling the requisite number of blade elements rigidly on a hollow central element having an outer surface adapted for mating relationship with the inner surfaces of each root section.

References Cited in the tile of this patent V 1 UNITED STATES PATENTS 1,010,929, Loetzer Dec. 5, 1911 1,973,131 Walgren Sept. 11, 1934 2,689,617 Bouley Sept. 21, 1954 FOREIGN PATENTS 179,381 Great Britain May 11, 1922 118,339 Sweden Mar. 11, 1947 

